This proposal covers two broad research areas: the ontogeny and complex usage of communication signals by monkeys and social and physiological influences on reproductive development and parental care in monkeys. Applications of techniques for studies of vocal communication and reproductive biology in captive primates will be applied to wild popu- lations. Studies of communication will focus on ontogenetic processes and will examine (1) ontogeny of structure and usage of food-associated calls in tamarins and pygmy marmosets, (2) ontogeny and function of "babbling" displayed by young marmosets, (3) convergence of call structure as pygmy marmosets are paired with each other, (4) ontogeny of structure and usage of coo calls given by infant rhesus monkeys during brief separations from mothers, (5) changes with age in usage of different call types with different social contexts in rhesus macaques. Studies of the social and physiological influences on reproductive behavior will focus on determining (1) the effective cue in pheromonal suppression of fertility in young female cotton-top tamarins, (2) the effective chemical component that allows male tamarins to detect ovulatory state in scent marks of females, (3) the mechanisms of male reproductive maturation and reproductive suppression, (4) the changes in response to separation and in response to same-sex and opposite sex in- truders during the development of a pair-bond, (5) how monkeys paired for the first time communicate sexual receptivity differently than well established pairs. The social and physiological aspects of male parental care in marmosets and tamarins will be studied by (1) developing a urinary prolactin assay that will be used longitudinally to determine if elevated prolactin levels are a cause or consequence of infant care, (2) to monitor the parental behavior of the adult male and of other helpers during the postpartum period to see if the adult male carries infants more at the time of his mate's postpartum ovulation and if a male obtains more mountings and copulations while he carries an infant. The non-invasive hormonal techniques and ideas developed from captive studies will be used in field studies on social and hormonal maturation of wild male mountain gorillas and in studies of mating, timing of postpartum estrus and infant care in wild cotton-top tamarins.